1. Field of the Invention
The present invention relates to an optical member configured to provide antireflective performance, an optical system including the optical member, and a method for producing an optical member. In particular, the present invention relates to an optical member configured to stably provide high antireflective performance from the visible region to the near-infrared region over a prolonged period of time and an optical system including the optical member.
2. Description of the Related Art
Antireflective structures with periodic microstructures which have repetition periods each equal to or lower than a wavelength in the visible region and which have appropriate pitches and heights are known to provide excellent antireflection performance in a wide wavelength range. A known example of a method for forming a periodic microstructure is a method in which a film including fine particles dispersed therein is formed by coating, the fine particles each having a particle size equal to or lower than a wavelength in the visible region.
A microfabrication method for forming a periodic microstructure by patterning with micromachining equipment, e.g., an electron beam lithography system, a laser interference exposure apparatus, a semiconductor exposure apparatus, or an etching apparatus, enables us to control the pitch and height of the periodic microstructure. It is also known that the microfabrication method provides the formation of a periodic microstructure having excellent antireflection performance.
Japanese Patent Laid-Open No. 9-202649 discloses another method in which a textured structure composed of boehmite, which is aluminum hydroxide oxide, is grown on a base to provide an antireflection effect. In this case, an aluminum oxide film formed by vacuum deposition or a liquid-phase method (sol-gel method) is subjected to steam treatment or hot-water immersion treatment to modify its surface into a boehmite surface having a plate-crystal structure, thereby providing an antireflection coating.
Although such a method for forming a plate-crystal film by steam treatment or hot-water immersion treatment is simple, a base needs to be exposed to steam or hot water. So, if the base is composed of glass, some constituents may be leached during the immersion in hot water to preclude the growth of the plate-crystal structure or to be left in the textured structure, thereby reducing the antireflective performance.
Furthermore, even if glass, whose constituents are likely to be leached, is just allowed to stand for prolonged periods of time at high temperature and humidity, the constituents may be leached out to reduce the performance of the plate-crystal film. To prevent leaching of constituents from glass, a technique using a laminated film having a diffusion preventing function is disclosed. For example, Japanese Patent Laid-Open No. 2006-259711 discloses that an antireflection coating in which a film having an intermediate refractive index is arranged between a base and a plate-crystal film (boehmite film having a textured structure) provides relatively stable antireflective performance.
For optical members, such as lenses, various improvements are made in order to prevent the generation of harmful light that causes flare and ghosting and to provide high-quality, high-performance optical members. For example, the arrangement of a light-shielding film on a ray ineffective area of an optical member makes it possible to improve the absorptance of harmful light based on the optical design to reduce the reflectance. In this case, a method for forming a light-shielding film by coating on a ray ineffective area, such as a side end portion, of an optical member is employed.
Japanese Patent Laid-Open No. 58-004946 discloses a light-shielding film made of an inner-surface-reflection-preventing coating material that contains a vinylidene chloride-based copolymer of a vinyl ester or acrylonitrile, and either coal tar or coal-tar pitch. The light-shielding film has excellent antireflective performance at the end face of a lens.
As described above, while an antireflection coating is required to provide excellent antireflective performance, the related art has problems described below.
In the case where a protective film is formed in a ray effective area, a light-shielding film is located in a ray ineffective area of an optical member. This is liable to cause uneven thickness or nonuniform treatment in the ray ineffective area, thereby reducing the antireflection function of a plate-crystal film having a boehmite textured structure configured to prevent reflection in a ray effective area. As a result, a region of the ray effective area where reflection is prevented is reduced, thereby leading to poor appearance and an increase in the reflectance of the optical member.
A light-shielding film located in a ray ineffective area of an optical member can be delaminated to cause poor appearance in long-term use or a high-temperature and high-humidity environmental test.
Furthermore, after the long-term use or the environmental durability test, the leaching of a constituent from a base of an optical member can change the optical properties of a plate-crystal film arranged to prevent reflection.
The present invention has been accomplished in light of the circumstances described above. Aspects of the present invention provide an optical member having stable antireflective performance for prolonged periods of time, an optical system including the optical member, and a method for producing an optical member.